1. Field of the Invention
This invention relates generally to refrigeration systems. More specifically, the invention relates to the control of an electricallydriven expansion valve for use in a refrigeration system.
2. Description of the Prior Art
Refrigeration systems include an expansion valve which regulates the flow rate of a refrigerant flowing in the system. It is known to change the degree of opening of the expansion valve in accordance with the heat load of an evaporator of the system such that the degree of superheat (SH) of the refrigerant at the outlet of the evaporator is held constant.
A refrigeration system usually exhibits an unstable transient state immediately after it is started. Therefore, it is very difficult to achieve accurate superheat (SH) control by regulating the degree of opening of the expansion valve.
When accurate superheat (SH) control cannot be achieved, an excessive degree of superheat (SH) may occur due to either too low a refrigerant flow rate, or "liquid back" which is a return of a liquid refrigerant to the compressor due to an excessive flow rate. When the flow rate is either too low or too high, performance and reliability of the system are degraded. The ability of the system to cool is lessened. Power consumption of the compressor increases, and the compressor abnormally overheats.
Japanese Utility Model Laid-Open No. 60-146267 discloses a refrigeration system which uses an electrically-driven expansion valve the degree of opening of which can be electrically controlled. The system determines the deviation, at the outlet of the evaporator of the refrigeration system, between the actual degree of superheat and a desired (objective) degree of superheat and controls the degree of opening of the expansion valve accordingly. Control of the expansion valve is carried out in accordance with a predetermined function of the deviation. This predetermined function includes proportional, integral, and derivative control components (hereinafter abbreviated as PID control). A control constant (control gain) of the PID control is changed after a predetermined elapsed time from the start of the refrigeration system (set time). In other words, it is pre-programmed that after some fixed specified time, the gain of the PID control circuit will be changed from a start value to a steady state operation value.
The actual operating condition of the refrigeration system changes largely due to a variation of the rotating speed of the system's compressor. In particular, in the case of an automobile air conditioner, the rotating speed of the compressor varies over a wide range of rotational speeds because it is driven by an engine. The actual operating condition is also affected by the amount of heat load, etc. Therefore, the time interval from the start of the refrigeration system to the moment the objective degree of superheat is reached varies largely depending on the operating condition. Consider a system which is started (initially turned on) when the expansion valve is set to a predetermined degree of opening, for example, it is fully opened. For a high heat load, the expansion valve can reach its adequate degree of opening within a short time from the start from start up of the system because the desired degree of opening is close to full-open. In contrast, for a low heat load, the desired degree of opening of the expansion valve is small. The degree of opening must be decreased from the initial full-open state down to a nearly-closed state. Therefore, the time interval required for the expansion valve to reached the desired degree of opening is long in comparison with the case of a high heat load.
If the PID control constant is changed from a first value which is suitable for a period immediately after the start to a second value that is suitable for steady state operation after only a short time (which can be suitable in a high heat load), this change of value of the PID control constant is not suitable for low heat load operation because the degree of opening has not been sufficiently decreased before this change. This makes the time required for reaching an adequately-controlled superheat state longer than it should be.
If the time interval from system start to when the time constant change of the PID control circuit is long (which can be an appropriate choice for low load steady state operation even though the correct degree of opening can be reached within a short time), this time interval would not be suitable for a high-load state condition. The system would tend to "hunt".
Since the above-described known refrigeration system varies the PID control constant after a predetermined elapsed time after the start of the system, if the system is designed so that the expansion valve can be controlled adequately for one case (high evaporator heat load), or for the other case (low evaporator heat load), adequate control cannot be achieved in the opposite case not taken into consideration in design. The system can be designed only for one of the two cases.